This invention generally relates to stabilizing compositions of matter, compositions comprising stabilizing compounds and processes for preparing stabilizing compounds and compositions. More particularly, the invention relates to polymeric compositions which are stabilized against the effects of oxidation or aging by the incorporation of phenolic antioxidant monomers into the polymer to be stabilized or into compatible stabilizing copolymers.
A wide variety of oxidation stabilizers have been used for polymer-containing compositions, including amines, phenols, phosphites, sulfides, and metal salts. Hindered phenol antioxidants are frequently preferred because of their relatively slight tendency to stain or discolor. Compatibility with the variety of systems to be stabilized against oxidation has become an important factor. There is the need to overcome the tendencies of the antioxidant to migrate, evaporate, or separate from such systems as solid polymers or to form localized concentrations, especially when processing involves heating, melting or exposure to leaching conditions like dry cleaning solvents, water, washing, weather or other forces which would encourage depletion of the antioxidant from the protected system.
To control migration, evaporation and leaching in polymer systems it has been considered desirable to bind the antioxidants into the polymers either by copolymerization or, more often by grafting. It has, however, proven difficult to retain the activity of the monomer when it is polymerized, or to include enough antioxidant in the copolymer or graft polymer to achieve the desired stabilizing effects. Antioxidants like hindered phenols tend to be free radical scavengers or inhibitors; thus, they tend to inhibit free radical polymerization. Producing copolymers of hindered phenol monomers has often proven complex and expensive. Addition copolymers of most hindered phenol antioxidant monomers have been limited to less than about 10 percent antioxidant monomer (see U.S. Pat. No. 3,627,831, H. Huber-Emden, et al., Dec. 14, 1971; and Rubber Chem. Tech., 46, pp. 96-105, 106-114 (1973); R. Levy, Rev. Gen. Caout, Plast., 51(4), pp. 243-247 (1974).
Various approaches to forming the desired polymers with antioxidant properties while avoiding their polymerization inhibiting effects have included grafting or otherwise reacting antioxidant compounds with already formed polymers, blocking the free radical inhibiting functional groups during polymerization and later removing the blocking group, or using catalysts which do not require free radical formation, such as transition metals, acids or base catalysts. Some approaches have involved carbene or chain transfer mechanisms that resulted in the phenolic ring being part of the polymer backbone rather than being an active pendant group.
Only a small portion of the approaches to solving the problems of forming antioxidants in polymeric form involve the use of antioxidant monomers having addition polymerizable unsaturation because these compounds have been difficult to make and purify as well as to polymerize in concentrations sufficient to impart antioxidant function to systems in which they might be used.
Concentrations of active antioxidant monomers in copolymers in excess of 10 percent would be useful not only to protect the polymer chains in which they are incorporated against oxidation, but also to protect other components in systems in which the antioxidant copolymers might be incorporated. They would be particularly useful in forms compatible with the systems to be stabilized against oxidation, especially in latex compositions which are sensitive to separation and other effects from the addition of incompatible compounds. Emulsion polymerization would produce such compatible forms of antioxidant polymers and would allow a high solids content and lower viscosity (therefore, easier handling) than polymers produced in solution systems. Unfortunately, it has proven particularly difficult to incorporate the needed quantities of antioxidant monomers in polymers produced in emulsion systems.
Solution polymerization systems in which copolymers containing at least about 10 percent antioxidant phenol monomers have been reported to involve heterocyclic, carbonyl, amide, acid, ester or other electron-attracting or conjugating functional groups directly on or in conjugational relationship to the phenol ring. Specifically, benzoic and cinnamic acid derivatives, benzophenone compounds and acrylamido-, acryloxy-, and acryloyloxy-phenols have been used. Benzoate diesters with unsaturated acid monomers containing hindered hydroxyl groups have been reported to be polymerizable in emulsion systems (U.S. Pat. No. 3,645,970, E. K. Kleiner, Feb. 29, 1972). It is known, however, that electron-attracting groups decrease the efficiency of hindered phenol antioxidants. See Encyclopedia of Polymer Science and Technology, Vol. 2, p. 176 (1965).
In U.S. application Ser. No. 298,688, filed Sept. 2, 1981, Mark R. Johnson taught an improved method of producing an addition polymerizable antioxidant with high activity, which need not have electron-attracting groups on the phenolic ring. In the pertinent part, this process employs an unsaturated isocyanatoalkyl ester reacted with an active hydrogen of an antioxidant compound to form a urethane type of linkage. The reaction product has unsaturation for addition polymerization as well as the active antioxidant functionality. Even that application did not disclose how to incorporate high levels of antioxidant monomers.
Unexpectedly, it has been observed that the inclusion of at least one unsaturated carboxylic acid monomer in a copolymer facilitates incorporation of antioxidant monomers into the copolymer, especially when the copolymer is produced in an emulsion system. Surprisingly, relatively high proportions, specifically at least 10 percent by weight, of antioxidant may be included in the copolymer chain using the same polymerization techniques that have proven effective for monomers which do not inhibit oxidation or polymerization when at least 15 percent by weight of an unsaturated carboxylic acid monomer is used. The antioxidant monomers may be present in amounts from 0.01 to 50 weight percent.
Such a result is particularly surprising in light of the fact that phenols, particularly branched lower alkyl substituted phenols are known to inhibit the polymerization of acrylic acid. See U.S. Pat. Nos. 3,888,927, L. B. Levy and G. F. Fisher, June 10, 1975 and 4,267,365, K. Findelsen, May 12, 1981.
When unsaturated active phenol antioxidant monomers have been incorporated into polymer systems (albeit in low concentrations) the polymer systems have generally been rubber systems like styrene-butadiene. Unsaturated acid monomers have not generally been present. Some examples in U.S. Pat. No. 3,627,831 did include about 5 percent acrylic acid, however.